In recent years, as an example of a technology for improving a data transmission rate, Discrete Multi-Tone (DMT) modulation has been put into practical use. DMT modulation is one of a multicarrier transmission schemes, and data is transmitted by using a plurality of subcarriers. Namely, in DMT modulation, a plurality of subcarriers are allocated to one signal band. Data is transmitted by using respective subcarriers. Accordingly, by allocating a large number of subcarriers to one signal band, a large capacity of data transmission is achieved without increasing transmission rates of the respective subcarriers.
In a DMT transmission system, DMT negotiation is performed between a pair of transmission stations before data transmission is started. In DMT negotiation, the quality of each of the subcarriers in a DMT signal is detected, and the number of bits to be allocated to each of the subcarriers in the DMT signal is determined in accordance with the detected quality. At this time, a modulation format of each of the subcarriers in the DMT signal is determined. Consequently, efficient data transmission is achieved without deterioration of the communication quality of each of the subcarriers.
An automatic negotiation system of a communication rate that realizes data transmission in which data is not discarded between terminal devices that are connected to each other via an optical fiber has been proposed (for example, Japanese Laid-open Patent Publication No. 2006-135487). In addition, Japanese Laid-open Patent Publication No. 9-191291 describes a related technology.
Another example of a technology for achieving an increase in capacity of a communication system, Wavelength Division Multiplexing (WDM) has been popular. In WDM, data is transmitted by using a plurality of wavelengths different from each other. Stated another way, in WDM, a plurality of wavelength channels are multiplexed. By applying DMT modulation to the respective wavelength channels in WDM, further speeding-up or an increase in capacity of the communication system is achieved.
In a system in which DMT modulation is applied to respective wavelength channels in WDM, the number of subcarriers used to transmit data inevitably increases. However, in order to realize efficient data transmission, DMT negotiation needs to be performed on respective subcarriers, as described above. Accordingly, in the system in which DMT modulation is applied to respective wavelength channels in WDM, the time needed to perform DMT negotiation that is performed before data transmission is started increases. Note that this problem does not occur only in a DMT transmission system, but this problem may occur in an optical transmission system in which multicarrier modulation is applied to respective wavelength channels in WDM.